Television transmitter



Jully 2U, 19430 J. R. PIERCE TELEVISION TRANSMITTER Filed Sept. 30, 19412 Sheets-Sheet l COLLECTOR T DEFLECTION CIRCUITS H H c I A s 0 M 2 m E NA L P G N 0 L A I E c N n S 0 DISTANCE ALONG PLANE 0F MOSAIC lNVENTOR.J./?.P//?C BY 4% 1M? ATTORNEY Patented July 20, 1943 TELEVISIONTRANSMITTER John R. lPierce, New York, N. Y., assignor to Bell Telephonelaboratories, Incorporated, New llorlr, N. ll., a corporation of NewYork Application September 30, 1941, Serial No. 412,937

18 Claims.

This invention relates to electron discharge devices and morespecifically to electron camera tubes for television and to methods ofoperating tubes of this type.

There has already been devised a television camera tube of the type inwhich an image of the picture to be transmitted is formed upon a mosaicof minute light-sensitive elements supported on and insulated from acommon metallic back-plate. Each element is the cathode of aphotoelectric film, a collecting electrode within the tube common tothese elements forming a common anode for the array of small cathodes.Each of the photoelectric cell cathodes acquires a potential, aselectrons are emitted therefrom to the collecting electrode, which has avalue depending upon the intensity or" the light striking it. The mosaicis scanned by a cathode ray beam whereby picture current appears in anoutput circuit, the picture current having a value depending on saidpotentials of the light-sensitive elements. The sensitivity of thiscathode ray tube is improved by applying a negative polarizing voltageto the light-sensitive elements by providing the tube with an awnliarysource of electrons and spraying these electrons upon the light-senstivemosaic. The electrons are projected towards the mosaic at acomparatively low velocity, that is, lower than the velocitycorresponding to some potential V at which one electron striking themosaic surface will on the average cause less than one electron to leavethe surface. The. value of V0 is dependent upon the specific materialused in the mosaic; for the usual cesiated surfaces V0 is of the orderof to volts, but in some cases it may be as high as 65 volts or more. Astream of such low velocity electrons with an energy less than V0striking the mosaic surface will result in an accumulation of electronson the surface and a lowering of potential of the surface. Electrons maybe sprayed upon the mosaic continuously or may be appliedintermittently, as at the end of each horizontal scanning line, Such acamera tube is disclosed in Patent 2,147,760, issued February 21, 1939,to Vance et al.

In accordance with the present invention. which is an outgrowth of aninvestigation of camera tubes of the type above described, there isprovided a tube in which the low velocity electrons produce a greatlyincreased reduction of the potential of the target, thus materiallyincreasing the sensitivity obtainable in this type of tube.

In an arrangement of the type described above, as the electrons from theauxiliary gun strike an elemental region of the mosaic, the potential ofthat region will be lowered gradually. It is obvious, from the law ofconservation of energy, that the potential of the region cannot becomelower than the potential of the cathode of the auxiliary gun, which isat a potential V1 with respect to the potential of the final anode inthe primary electron gun (the collecting electrode, generally) and alsowith respect to the final anode of the auxiliary gun. However, if theelectrons are projected towards the mosaic at a considerable angle tothe normeal to the mosaic, the lowest negative potential the mosaic canattain with respect to the final anode potential is considerably lessthan V1, or in other words, the effectiveness of the low velocityelectrons is lowered, Just how low a potential can be attained dependson the angle of projection and on the exact nature of the field betweenthe mosaic and the collecting electrode (the final anode of the primarygun). It has been found that, for the case where there is a uniformfield between the mosaic and the collector electrode, electrons cannotstrike the mosaic if it is at a lower potential with respect to thecollecting anode than V1 cos 0, where 0 is the angle that the electronbeam from the auxiliary electrode makes with the normal to the surfaceof the mosaic. Gonslderable departure from exact uniformity of the fieldmay be present Without much change in this relationship. In the Vance eta1. arrangement, the electrons strike the center of the mosaic from alow velocity gun at an angle, 0, which appears t be approximately 52degrees, so that cos 0 is approximately 0.37 while for electronsstriking the mosaic, near the remote edge, 0 is approximately degreesand cos 0 approximately 0.23. As explained above, it has been found.that this places an unnecessary limit upon the negative potential whichcan be given to the mosaic. Moreover, because of the angle of approachof the auxiliary electrons a shift in the position of the low velocitybeam occurs during operation which is likely to produce spurious effectsin the signal. This is further discussed below.

On object of this invention is to provide an improved method ofoperating an electron discharge device to largely, at least, overcomethe above-mentioned disadvantage.

It is also an object of this invention to provide an improved electrondischarge device which overcomes, at least in large degree, theabove-mentioned disadvantages.

only 7 per cent less than that produced when cos 9 is unity.

In accordance with an illustrative embodiment of the present invention,there is provided an electron discharge device having a mosaic targetupon which an optical image is projected, means for scanning the mosaicwith a beam of primary electrons, and a plurality of supplementaryelectron guns for generating beams of such low velocity that thesecondary emitting ratio of the material for that velocity is less thanI. These supplementary guns are so disposed that a large portion, atleast, of the electrons therefrom reach the mosaic target as nearly aspossiblenormal to the plane of its front face. The guns are preferablymade so that they produce an electron beam which has a considerableextent in one direction; for example,

the beam when it strikes the target may have a dimension in onecoordinate direction corresponding to the width of the target but have adimension in the other coordinate direction of approximately one half ofthe mosaic dimension in this direction. Each supplementary electron gunthus forms a wedge-shaped beam which resembles somewhat the partiallyopened leaves of a book. With this type of gun, electrons obviouslyreach various portions of the mosaic more nearly normal to the surfacethan is the case with a gun giving a conical beam. Moreover, this solvesthe problem of. proper overlapping of beams which is raised with the useof two or more guns.

While two, four, six or any number of auxiliary guns may be used, therehas been shown, by way of example for purposes of illustration, anarrangement in which four low velocity guns are disposed within thecontainer so that their beams are directed upon the mosaic target insuch a manner that the axes of said beams are, as nearly as possible,normal to its surface. Two of the guns may have the long dimension oftheir beams parallel to one dimension of the mosaic as, for example, theheight, while the other two guns may have the long dimensions of theirbeams parallel to the other dimension of the mosaic as, for example, thewidth. In one form of this arrangement, two of the guns generate anabundance of very low velocity electrons, while the other two generate asmaller current of fairly low velocity electrons, but not as low as thevery low velocity electrons. By wayof example, the very low velocityelectrons may be from 1 to 5 volts while the fairly low velocityelectrons may be of from 10 to 20 volts. The very low velocity electronsbring the mosiac to a potential which is negative with respect to thecollecting electrodes a very short time after scanning. As the mosaicelements gradually become more negative, the electrons from this gun nolonger reach the mosaic and have no further effect. The current from thefairly low velocity electron beam generating means, however, althoughsmaller in intensity, acts at all times. The cathodes of the auxiliaryguns may be hid from the mosaic by novel electron gun construction sothat the material of the cathodes cannot evaporate onto the mosaic.

The invention will be more readily understood by referring to thefollowing description taken in connection with the accompanying drawingsforming a part thereof in which:

Fig. 1 is a schematic view of a television transmitter system inaccordance with the invention;

Fig. 2 is a partial side elevation view of the tube shown in Fig. 1 withportions thereof broken away;

Fig. 3 is a somewhat larger view of a supplementary electron gun whichmay be used in the tube shown in Fig. 1; and

Figs. 4 to 8, inclusive, are graphical and diagrammatic representationsto aid in explaining the invention and the operation thereof.

Referring more specifically to the drawings, there is provided, by wayof example to illustrate the principles of this invention, a televisiontransmitting system employing an electron discharge device l0 togetherwith certain of its associated circuits. The tube l0 preferablycomprises a highly evacuated envelope H enclosing a primary electron gunG consisting of a cathode i2, a cathode heater 13, a modulatingelectrode M, a first anode I5, a second anode l6, and a third anode II,two pairs of electrostatic defleeting plates i8, i8 and l9, IS, a finalanode 33 (the collecting electrode) which is preferably in the form of aconducting coating on a portion of the walls of the tube, a mosaictarget 20, and a plurality of supplementary electron guns G1, G2, G3 andG4 for generating and projecting upon the target low velocity electrons.

Referring first to the primary gun G, current for the heater I3 issupplied by a source of potential 2|. The cathode I2 is connected to thenegative terminal of a source 22, the positive terminal of which ispreferably connected to the second anode IS. The first and third anodemembers l5 and I1 and the final anode member 33 are connected to thepositive terminal of a source 23 which positive terminal is preferablyconnected to ground. The negative terminal of source 23 is connectedthrough the sources 24, 25 and 26 to the positive terminal of the source22. The negative terminal of the source 22 is connected to the positiveterminal of the source 21, the negative terminal of which is connectedto the modulating electrode I 4. While separate sources have been shownfor convenience, it is obvious that the sources 22 to 21, inclusive, maybe a single source, such as a potentiometer resistor the terminals ofwhich are connected to any suitable source of direct current, such as arectifier or battery. By means of this arrangement, the highestpotential of the tube is ground potential, which is that of the anodemembers I5, H and 33. This potential is from 1000 to 2000 volts positivewith respect to cathode potential which is thus 1000 to 2000 voltsnegative with respect to ground. The modulating or control electrode I4is placed at a potential which is negative with respect to the cathode.The potential of the electrode I6 is somewhere between the potential ofthe cathode and the potential of the electrode members I 5 and I1 and isadjusted to give the desired current. The exact potentials of thevarious electrode members are chosen in accordance with well-knownelectron lens practice so that the beam of electrons is brought to asharp focus at the mosaic target 20.

The mosaic target 20, which per se is not part of the present invention,may be constructed in a variety of ways. In a preferred form it consistsof a thin sheet of mica or glass 34 having a continuous metallic coatingon the back thereof ,which is connected by means of the lead 3| to anoutput resistor 32, the other terminal of which coatingis connected tothe conducting coating 39 in the tube. The metallic coating 30 will bereferred to as the signal plate. The mosaic proper may consist of amultitude of silver globules each of which has a film of silver oxidethereon, the silver oxide film being coated with caesium. In such amosaic, the light-sensitive elements are electrically insulated fromeach other and from the metallic coating or signal plate 39 on the backof the mica or glass sheet 3 1. Another possible method of constructinga suitable mosaic is that of forming a light-sensitive layer upon thefront side of the mica and separating the light-sensitive material 35into a by the electron gun apparatus described above,

to scan every elemental area of the image or field of view on the mosaictarget 29 in turn, suitable deflecting means such as, for example, twopairs of deflecting plates l8, l8 and l9, t9, the axes of which arelocated at right angles to each other, are provided. To the deflectingplates 99, i9 are applied deflecting voltages of framing frequency andhaving a saw-tooth wave form to produce the vertical deflection of thebeam while deflecting voltages of line scanning frequency andofsaw-tooth wave form are applied to the deflecting plates 18, iii toproduce the horizontal deflection of the beam. Any suitable sweepcircuits (not shown) may be used to generate these horizontal andvertical deflecting voltages. For example, Patent 2,178,464., issuedOctober 31, 1939, to M. W. Baldwin, Jr., discloses appropriate balancedsweep circuits for this purpose. Connections may be made from thebalanced sweep circuits to the pairs of plates l9, l8 and i9, 9 by meansof high resistance coupling resistors l t and 75, respectively connectedacross the pairs of plates l8, l8 and l9, H9. The mid-points of theresistors M and 715 are connected to ground in order that the averagepotential of the defleeting plates is at all times substantially equalto the potential of the final anode which, in the arrangement shown, isthe potential of the electrodes l5, H and 33. For a full description ofthe advantages of balanced sweep circuits, reference may be made to theabove-mentioned Baldwin patent and also to Patent 2,209,199, is-

sued July 23, 1940, to Frank Gray.

Outside the cathode ray tube is an optical system representedgenerallyby the single lens til for projecting an image of an objectupon the photosensitive surface 35 of the mosaic target 25. The tube Wis so arranged that the axis of the radiations striking the target 20from the object or field of view is substantially normal to the surfaceof the mosaic, as will be pointed out more fully below.

Also arranged within the container it are four auxiliary electron gunsG1, G2, G3 and G4. As shown in Fig. 2 which is a side elevational Viewof a portion of the tube shown in Fig. 1 with parts of the walls of theenvelope I l broken away to show the interior of the tube, the four gunsG1, G2, G3 and G4 are arranged so that they are not in the path of thelight from the object. These guns are preferably of a type to produce along wedge-shaped beam. The four auxiliary guns generate beams of lowvelocity, that is, not exceeding 20 volts and may be grouped into afirst group comprising, for example, the guns G1 and G2, which are usedto spray a small quantity of 10 to 20 volt electrons ,upon the mosaictarget 29 and a second group,

' iliary guns G1 to G4, inclusive, it seems advisable to discuss theconsiderations which led to the present invention. First of all, itshould be understood that an electron striking the mosaic surface withan energy lower than that corresponding to some potential V0 will on theaverage cause less than one electron to leave the surface. On the basisof experience with cesiated surfaces, V0 is usually of the order of 10or 20 volts, although several experimenters suggest that Vo may have avalue as high as 65 volts. It will be apparent, however, that the scopeof the invention is not limited to an exact value of V0.

A stream of such electrons with an energy less than V0, striking themosaic surface 29, will thus tend to result in an accumulation ofelectrons on the'surface and a lowering of potential of the surface. Onthe other hand an electron striking the surface with an energy somewhatgreater than that corresponding to the potential V0 will on the averagecause the emission of more than one electron. Reference will now be madeto Fig. 3 which shows one type of low velocity auxiliary electron gunwhich may he used in this invention, although it is to be understoodthat the invention is not limited to any particular type of auxiliaryelectron gun, the arrangement in Fig. 3 being merely a preferred form.The low velocity gun shown in this figure comprises a cathode 59 whichis preferably,

a long strip, a first accelerating anode 5i and a final acceleratingelectrode 52. A. deflecting electrode "it, which is curved so that theelectrons are caused to be deflected in such a manner that the axis ofthe deflected electrons is approximately at right angles to the plane ofthe emitting surface of the strip cathode i 5, is connected at cathodepotential and is placed between the anode members 5! and 52. In thearrangement shown in Fig. l, the final accelerating anodes 52 of allfour guns G1, G2, G3 and G1 are preferably connected to ground by meansof the connections 5t, 54 and 55. The first accelerating electrodes 5iof the tubes G1 and G2 are connected by means of the leads 56, 57? and55 to the common terminal of the sources 25 and 25. The cathodes 59 ofthe tubes G1 and G2 are preferably connected through connections 59, 59and ill to the common terminal of the sources 25 and 26 which placeseach cathode at a potential of from 10 to 20 volts negative with respectto the potential of the final accelerating electrode 52, or in otherwords, the final electrode 52 is from 10 to 20 volts positive withrespect to the potential of the cathode.

final accelerating electrodes 52, the first accelerating' electrode andthe cathode, respectively, of the auxiliary guns Ga and G4 which aresimilar to the guns G1 and Ga. By this means a positive potential offrom 1 to 5 volts with respect to the potential of the cathode is placedupon the final accelerating electrodes 52 of these two guns Ga and G4inasmuch as the lead 64 is connected to the common terminal of thesources 24 and 25 while the lead 62 is connected to ground. The lead 63is connected to the common terminal of sources 23 and 24 and thus isplaced at a potential which is intermediate ground potential and thepotential of the lead 33, thus making the potential of thefirstaccelcrating member SI of the guns Ga and G4 intermediate that ofthe cathode 50 and the final accelerating electrode 52. While it is notabsolutely necessary that the potential of each member 5i beintermediate that of each of members 50 and 52, this is the more usualarrangement.

The four guns G1, G1, G3 and G4 are preferably arranged symmetricallyabout the normal to the mosaic through the center of the scanning fieldthereon. The currents of the auxiliary guns are controlled by varyingthe potentials of the intermediate electrodes 5i with respect to thepotentials of the cathodes 50 as indicated schematically in Fig, 3 whichshows the cathode 50 of the gun in that figure connected to the negativeterminal of a source 80 and the potential of the final electrode 52connected to the positive terminal of the source 80, a variable tap 1|being connected to the first accelerating electrode 5|. The curveddeflecting member 10, as indicated above, is preferably at cathodepotential. For simplicity in the drawing, in the guns shown in Fig. 1the members have been omitted. Heaters for the cathodes have not beenshown but it is ob vious that any suitable heater may be provided.

In the arrangement above described, the very low velocity guns G3 and G4produce a large quantity of electrons which brings the mosaic 20 to apotential which is negative with respect to the collecting electrode 33a very short time after a particular elemental area under considerationhas been scanned. As the mosaic elements become gradually more negativewith respect to final anodes 52 they approach the potential of the finalanodes of the guns Ga and G4, and electrons from these guns no longerhave energy enough to overcome the decelerating field between anodes 52and the mosaic and can no longer reach the mosaic. The cathodes of gunsG1 and G2, which are from 10 to 20 volts negative with respect to theanodes 52, are still negative with respect to the mosaic, and henceelectrons from these guns continue to reach the mosaic, graduallylowering its potential.

By means of this invention, the iconoscope mosaic is made negative withrespect to the collector electrode 33 during most of the cycle ofoperations as the mosaic is sprayed with a steady stream of electronsfrom the guns G1, G2, G3 and G4 or at least some of them, and by makinguse of several guns the whole target is sprayed uniformly, and moreover,in this arrangement the electrons from the auxiliary guns approach thetarget at an angle which is as near a right angle as it is possible tomake it without cutting off light from the object.

As pointed out above, the beams at the surface of the target shouldoverlap somewhat in order to improve the current density distributionover the front face of the target. The current density distribution ofthe beam of one gun, as forexample, gun G1, along a line on the targetin the plane of the drawing, may be represented by curve I in Figure '7where this current intensity dis-mi bution of the beam in arbitraryunits is plotted against the distance, also in arbitrary units, alongthe plane of the mosaic. A similar curve 2 for the beam of the gun G2 isalso shown in Fig. '7. The two curves l and 2 in Fig. 7 can be combinedto give the resultant curve shown in Fig. 8. As indicated by the longflat-top portion of the curve of Fig. 8, by using the proper amount ofoverlap, the current intensity distribution of the electrons from thesupplementary guns may be made substantially uniform over a relativelylarge portion of the mosaic surface. The degree of overlap necessarywill depend, of course, on the particular gun structure and tubegeometry. Overlapping beams from guns G1 and G2 have been indicated inFig. 1 but the beams from the guns Go and G4. which are also preferablymade to overlap, have been omitted in this figure but merely for thepurpose of simplifying the drawing.

In operation, the scanning current from the gun G should be adequate torestore each mosaic element to the equilibrium potential, slightlypositive with respect to the collector, after scanning. The currentproduced by the electron spray from the auxiliary guns G1, Ga, Ga and G4is such that between scans an unilluminated portion of the mosaicreaches a potential which is negative with respect to the collectingelectrode 33. An illuminated portion becomes less negative with respectto the collector because of the loss of photoelectrons between scans.Thus an unilluminated portion might become 8 volts negative -lust beforescanning, while an illuminated portion becomes, for example, 6 voltsnegative.

When it is scanned by the beam from the gun G, an elemental portion ofthe mosaic surface 35 is raised to a potential substantially equal tothe potential of the electrode 33 and the anode 52,

and, immediately after scanning, such portion of the mosaic surface canbe considered as being at the same potential as the anode 52. Underthese conditions, electrons emanating from the cathodes 50 of the gunsG1 and G2 will strike the mosaic with an energy corresponding to thepotential Vi supplied by that portion of the power supply consisting ofbatteries 23, 24 and 25. From the foregoing it can be seen that if theelectrons leaving the cathode 50 are to lower the potential of themosaic, as is desired, V1 must be less than Vo. the voltage for unitysecondary emission ratio. Thus an upper limit is placed on the allowablevalue of V1.

As the electrons from the cathode 50 strike an elemental region of themosaic, the potential of that region will be lowered gradually. Itisobvious from the law of conservation of energy that the potential of theregion cannot become lower than the potential of the cathode 50 which isat a potential V1 with respect to the electrode 33. However, if

- the electrons are projected toward the mosaic at a considerable angleto normal (as in the Vance et a1. arrangement), the lowest potential themosaic can attain with respect to the member 33 is considerably lessthan V1. Just how low a potential can be attained depends on the angleof projection and on the exact nature of the fields between the mosaicand the electrode 33. A simple case will be considered by way of examplebut the conclusions drawn will be very similar to those in many othercases.

The simple case considered will be that in which electrons are projectedtowards the mosaic at an angle 6 with respect to the normal and in whichthe electrode structure is such that any electric field between themosaic and the electrode 33 is substantially uniform and normal to themosaic surface. Fig. 4 is a schematic diagram representing thiscondition showing the mosaic surface 35, the electrode 33, and variouselectron paths, ab, ac'and ad from one of the auxiliary guns G1. G2, G3and G4 to the surface 35, the path ab also representing the axis of thebeam which is inclined at an angle 0 with respect to a normal to themosaic surface. As the electrons strike the mosaic 35, its potential islowered with respect to the member 33 and there is set up, due to thechange in potential, a force which acts on electrons approaching it,which force is toward electrode 33 and away from the mosaic 35. The

uniform field assumed has no component parallel to the mosaic surface.When leaving the gun the electron travelling between the gun and themosaic surface has a velocity component 27p parallel to the mosaicsurface and a component 'Dn normal to the surface. These Components aregiven by v,, ==1 /2 V sin 0 (l) 2 V cos 0 (2) n -1 m l where V is thepotential of the mosaic with respect to the member 33. From Equation 3tFv gvw w e /2 (V cos 0 V) (4) It is seen that for values of V greaterthan V1 cos 0, Dr: is imaginary, and thus it is clear that electronscannot strike the mosaic 35 if it is at a lower potential with respectto 33 than V1 cos 0, where, as pointed out above, V1 is the potential ofthe mosaic with respect to the cathode of the auxiliary gun and 0 is theangle the electron trajectory makes with the normal to the mosaicsurface. If the mosaic were at a potential lower than this, electronpaths would take the parabolic form shown in the path ad, shown in Fig.i. As

the supply of electrons from the gun G to the mosaic 35 is the meansdepended on for lowering the mosaic potential, it is further obviousthat the mosaic cannot attain a potential lower than V1 cos 0. It hasalready been explained that V1 cannot be greater than V0, the voltagefor unity secondary emission ratio. It thus follows that when lowvelocity electrons are used to lower the potential of the mosaic, themosaic 35 cannot be energy, the following rela greater than Vx whereVI=VO cos 0 (5) Vx can therefore be considered the maximum potential(negative with respect to the collecting electrode) that the mosaicsurface can reach for a given set of conditions (shape and intensity ofelectrostatic fields, potential of cathode of auxiliary gun with respectto the mosaic, angle of electron approach to the target withrespect tothe normal, material of mosaic, etc).

Now the operation of the present invention will be considered in thelight of this fact.

In Fig. 5, the potential V with respect to the electrode 33 of anelemental region of the mosaic has been plotted with respect totime,-the time between points i and l constituting a scanning interval.In Fig. 5a, the elemental area is assumed to receive little or no lightfrom the object. Immediately after this element has been scanned (pointi of Fig. 5a), V will be almost equal to zeroperhaps slightly positivebecause the energies of the secondary electrons caused by the highvelocity scanning beam enable them to reach 7 the electrode 33 under aslight retarding field.

Electrons reaching the region from the low velocity gun cause thepotential V to fall until the scanning beam again reaches the region,indicated by point 2, at which time the potential is Va. Then thepotential abruptly rises until at the end I of scanning of thiselemental region, indicated by point i, it has the same value that ithad at l.

Then low velocity electrons cause a fall of potential until the nextscanning, at 2, etc.

Fig. 5?) illustrates the conditions at a region illuminated by a stronglight. In this case, many photoelectrons leave the mosaic during thearrival of slow electrons, and V falls less rapidly, reaching a value Vb(indicated by points 2 and 2') Vb being less than Va.

Now consider a case in which electrons from the low velocity gun aredirected at the mosaic at a considerable angle 0 to the normal, so thatVx is considerably smaller than V0. Let us consider the case of a lowlight intensity, as in the case represented in Fig. 5a. The result isillustrated in Fig. 50. V starts to vary at about the same rate as inthe case shown in Fig. 5a. However, V cannot reach the potential Vawhich will give the correct signal, because Vx is smaller than Va. Hencethe response of the device is false for small light intensity.

It may be objected that this may be remedied by making the current fromthe low potential gun smaller, so that the potential does not fall sofast. Then for a weak light the potential would behave as shown in Fig.5d, V falling to some value V's which is smaller than Vx. However, asshown in Fig. 5e, the device so modified will not respond to the stronglight of Fig. 52), for the potential cannot be lowered by the lowvelocity electrons since more photoelectrons are produced than there arelow velocity electrons which arrive at the mosaic. Thus the modifieddevice will give a false response to high light intensity.

Summing up, it can be said that to get a correct response to as wide arange of light intensities as. possible, VX must be as large aspossible. Vx canot be larger than V0, the potential for unity secondaryemission ratio. Vx will be considerably smaller than V0 if the lowVelocity electrons are projected at the screen at a large angle withrespect to the normal. For example, in Fig. 2 of the Vance et al. UnitedStates Patent 2,147,760 for electrons striking the center of the mosaicfrom the low velocity gun 4|, 43, 45, is shown in the patent to beapproximately 52 degrees and cos 9 is thus approximately .37, while forelectrons striking the edge region of the mosaic remote from theauxiliary gun, 0 is approximately 60 degrees and cos 0 is approximately.23. Thus the device, and more particularly as regards the edge regionof the mosaic, will be incapable of reproducing faithfully the widerange of light intensities that can be reproduced by means of the deviceof this invention.

In accordance with this invention, 0 is made such a low angle withrespect to the normal (by positioning the axis of the auxiliary gun orguns as near the normal to the mosaic as it is possible to do so (thatVx does not differ greatly from Vi, or, that cos 0 does not differgreatly from unity. For instance, for an angle with respect to thenormal of 0:15 degrees, cos 0:.93, or Vi; is only about 7 per cent lowerthan V1. This is to be compared with the arrangement shown in Fig. 2 ofthe Vance patent where it can be seen by measuring that the electronsstrike the center of the mosaic at an angle of approximately 52 degrees,and thus cos 0:.37, which makes Vs far less than the value obtained inaccordance with this invention. In the arrangement according to thepresent invention, the angle between the electron path at the target andthe normal to the target at that point is made so small that the cosineof that angle is not less than 0.9 (the angle being about 26 degrees orless).

There is a further point about angular incidence of electrons upon atarget which is of importance. 'Work by various investigators hasbrought out that materials are better secondary emitters for glancinglyincident electrons; in other words, Vo will be smaller for electronspro- Jected at a considerable angle to the normal, as proposed by Vance,than for electrons projected substantially normal, as in this invention.As it is desired to make Vx as large as possible, and Vx cannot begreater than V0 cos 0, this tends to further limit the range of lightresponsiveness and hence the usefulness of an arrangement utilizingelectron incidence at a considerable angle to the normal.

The electron guns G1 to G4, inclusive, each preferably emits awedge-shaped beam, with rays somewhat like the partially opened leavesof a book. With this type of gun the electrons obviously reach variousportions of the mosaic more nearly normal to the surface than would bethe case with a gun giving a conical beam. Moreover, the problem ofproper overlapping of beams. which is raised with the use of two or moreguns, is satisfactorily solved. Conical overlapping beams give an areaof overlap of uneven width while wedge-shaped beams give a constantwidth area of overlap. The proper amount of overlap depends on howrapidly the current density decreases near the edge of the beam and canbest be defined as the amount of overlap which will make the currentdensity on the mosaic most nearly constant.

It is obvious that more nearly normal incidence of a larger portion ofthe whole group of electrons emitted from the auxiliary source can beachieved by the use of ,two or more auxiliary guns than by the use ofonly one gun. However, if different parts of the mosaic are to besprayed by different guns, and if the current density of the spray is tobe kept substantially constant over the mosaic surface as is desirablefor uniform operation of all portions of the mosaic, the

overlap of the beams must be carefully controlled, With conical beams itis obvious that the overlap will be variable in width and hence auniform current density cannot be maintained. With guns such as those ofthe present invention the beam boundaries will be substantially linear,and with the proper amount of overlap the current density can be keptsubstantially constant over the whole mosaic.

As mentioned above, one type of operation proposed calls for the use oftwo kinds of slow electrons; an abundance of very slow speed electrons,say, 1 to volt electrons, and a controlled current of ,faster slowelectrons, say, to volt electrons. The reason for the 1 to 5 voltelectrons will be apparent from Fig. 6, in which the potential of anelemental area or region of the mosaic is plotted versus time.

Fig. 6a is a digaram of the operation with the "fast" low velocityelectrons only spraying the screen. The response of the device dependson photoelectrons leaving the mosaic. Immediately after scanning anelemental area (see point ill in the figure), photoelectrons cannotleave the region being discussed because it is positive with respect tothe surroundings. Photoelectrons can leave the region only after thetime (as shown at point 82) when this region becomes negative and thefields are favorable for the departure of photoelectrons. Thus theregion is effective photoelectrically only for the period Ati betweenpoints B2 and 83, and not for the whole period At between the time thescanning beam leaves the elemental area and the time it strikes thatarea again.

Fig. 6b is a diagram of the operation with an abundance of slow lowvelocity electrons as well as a controlled current of faster, but stillrelatively low, velocity electrons. In this case the "slow" low velecityelectrons lower the potential of the region to a potential Vm (shown aspoints M and M and approximately the potential of the cathode of the"slow low velocity electron guns with respect to the electrode 33)almost immediately after scanning. Thus the period of time Ati when V isnegative, is greater than when only fast" low velocity slow electronsare used. When the mosaic potential reaches a potential which isapproximately that of the cathode of the guns emitting the slow" lowvelocity electrons, the electrons from these guns no longer strike themosaic but return to the collecting electrode or other electrodeelements at a more positive potential than the mosaic.

There is a further advantage in the use of nearly normal incidence. Withslanting incidence as illustrated in Fig. 4 and in the Vance patent,electrons similarly directed strike various parts of the mosaicdepending on the mosaic potential. For instance, ab represents a pathfor a small potential difference between the members 35 and 33, and acrepresents a path for a larger potential difference between the members35 and 33. it difficult to direct the beam properly at the mosaic, andmight result in non-conformities of operation. It is obviated by thesubstantially normal incidence of the arrangement of the presentinvention.

It is possible to estimate the desired steady current supplied at thepotential V1. Assume the time between scans is T and the capacitancebetween the face 35 and the back 30 of the mosaic 20 is G. Then tochange all elements of the This shifting of beam with potential makesAssume reasonable values mosaic by a potential (1V1 between scansrequires a current in Equation 6, C=l2,000 micro-microfarads, V1=15volts,

cz= T= second, then io=3.6 microamperesu,

A slightly greater current will be necessary to compensate for secondaryemission at less than unity ratio. V1 is somewhat less than the voltageof impact at which the secondary emission ratio of the mosaic is unity.The factor a is always less than one. The larger 11 is the larger thelight signal to which the device will produce a linear response, subjectto the limitation. however, that if on approaches too closely to unity,the current from the low velocity guns to the mosaic will decreasetoward the end of the period between scans because the mosaic willapproach too closely the potential of the cathodes of these low velocityguns and some electrons will be turned back, leading to non-linearity ofresponse. The larger the angle 0, the smaller or must be made (byadjusting the current flowing from the auxiliary guns) in order that noelectrons from the relatively high speed low velocity guns shall beturned away from the mosaic. Not every electron approaching the mosaicfrom the auxiliary guns strikes it-or, at least, some electrons strikingthe mosaic cause other electrons to leave it. Thus, if for everyelectron reaching the mosaic from these guns, m electrons leave (where ml), to cause a not current is to flow to the mosaic, the auxiliary gunsmust supply a current of i=iu/(1-'m).

With the arrangement described above, there is nothing gained byincreasing the beam current beyond that necessary to restore eachelement to a. uniform potential after scanning. If t is the secondaryemission ratio of the mosaic at the voltage of the scanning beam, theleast beam current which can do this is that which can produce a netelectron current equal to in away from the mosaic. Thus the minimumscanning beam current would be Assuming 6:5 and the value of in abovecomputed, then ib=0.9 microampere although it might be desirable to usea beam current somewhat larger than this. 7

While the use of two difierent types of supplementary or auxiliary gunsis preferable in order to obtain the advantage stated above, it is ofcourse obvious that all of them may be of one type, preferably of thetype employing the higher velocity. This velocity is neverhigh enough,however, to make their secondary emitting ratio greater than one.

Although the present invention has been described largely in terms ofvarious illustrative embodiments. it will be apparent to those skilledin the art that the invention and its various features are susceptibleof embodiment in a wide variety of forms within the spirit and scope ofthe appended claims.

What is claimed is:

1 The method of operating a cathode ray tube of the highly evacuatedtype having a mosaic of light-sensitive elements electrically insulatedfrom each other and from a supporting structure and having a collectingelectrode for electrons which is common to said elements, which methodcomprises scanning said mosaic-with a beam of high velocity electronsand spraying said mosaic with a stream of low velocity electrons, theaxis of said stream being at an angle with respect to the normal to themosaic surface which has a coosine falling within the range between 0.9and 1.

2. The method of operating a cathode ray tube of the highly evacuatedtype having a mosaic of light-sensitive elements electrically insulatedfrom each other and from a supporting surface and having a collectingelectrode which is common to said element, which method comprisesscanning said mosaicwith a beam of high velocity electrons, sprayingsaid mosaic with an abundance of very low velocity electrons, and alsospraying said mosaic with a smaller number of electrons having a speedwhich is low compared with the beam of high velocity electrons but whichis higher than that of the very low velocity electrons.

3. In a cathode ray tube of the highly evacuated type having a mosaic oflight-sensitive elements electrically insulated from each other and froma supporting surface and having a collecting electrode which is commonto said elements, a plurality of electron guns so placed with respect tosaid mosaic that it is sprayed with low velocity electrons from saidguns, which electrons.

approach said mosaic at an angle with respect to a normal to said mosaicat which the cosine. thereof is at least 0.9.

.4. In a cathode ray device, a mosaic of lightsensitive elementselectrically insulated from each other and from a supporting surface, acollecting electrode common to said elements, a plurality of electronguns so placed with respect to said mosaic that it is sprayed With lowvelocity electrons which approach said mosaic at an angle of greaterthan about 64 degrees with respect thereto, the cathodes of the gunsgenerating the low velocity electrons being'out of sight, of the mosaicso that material from the cathode cannot evaporate onto the mosaic.

5. The combination of elements as in claim 4 in which each of saidelectron guns comprises a deflecting electrode so that the electrons areemitted from each of said guns at an angle which is substantially atright angles with respect to the axis of the electrons emitted from thecathode of said gun,

6. In combination, an evacuated container having mounted therein amosaic target of lightsensitive elements electrically insulated fromeach other and from a supporting electrode. means in said container forgenerating a beam of high velocity electrons and for causing said beamto scan every elemental area in turn of a field on said mosaic, meansfor applying radiations from an object or field of view to said target,means within said container for collecting secondary electrons emittedfrom said mosaic target when impacted by primary electrons from saidbeam, and a plurality of supplementary electron guns disposed around thenormal to the center of the field on said target, said guns being soplaced that streams of low velocity electrons emitted therefrom impingeupon said target at an angle of greater than about 64 degrees withrespect thereto.

7. In combination, an evacuated container having mounted therein amosaic target of lightsensitive elements electrically insulated fromeach other and from a supporting electrode, means in said container forgenerating a beam of high velocity electrons and for causing said beamto scan every elemental area in turn of a field on said mosaic, meansfor applying radiations from an object or field of view to said target,means within said container for collecting secondary electrons emittedfrom said mosaic and target when impacted by primary electrons from saidbeam, and a plurality of supplementary electron guns disposed around anormal to the center of the field on said target, said guns eachgenerating a beam of low velocity electrons which impinges upon saidtarget and said supplementary guns comprising at least one gun from eachof two types, one type comprising a gun which generates a beam ofelectrons of very low velocity, and the other type comprising a gunwhich generates a beam of a velocity which is low compared with thevelocity of the beam of high velocity electrons but which is higher thanthat of the very low velocity electrons.

8. The combination of elements as in claim 7 in which the gun producingelectrons of very low velocity emits more electrons than those of thesecond type.

9. The combination of elements as in claim 7 in which the supplementaryguns comprise four guns which are so disposed around said normal thattwo guns of the first type are opposite each other and two guns of thesecond type are also opposite each other.

10. The-combination of elements as in claim 7 in which each of saidsupplementary guns comprises a strip cathode. a first accelerating anodeand a second accelerating anode.

11. The combination of elements as in claim '7 in which each of saidauxiliary guns comprises a strip cathode, a first accelerating anode anda second accelerating anode, the potential of said second acceleratinganode being at least substantially the same as that of the collectingelectrode in said container, the cathodes of the guns of the first typebeing placed at a potential which is from 1 to 5 volts negative withrespect to the potential of said collecting electrode and the cathodesof the guns of said second type being placed at a potential which isfrom to volts negative with respect to the potential of said collectingelectrode.

12. In a cathode ray device, a screen or target, four electron guns,each of which generates a wedge-shaped beam, and means for directing thebeams generated by said guns at said screen or target in such a way thatthe beams overlap at the target, the longitudinal axis of two of saidbeams each on opposite sides of said screen or target being generallyparallel to one axis of said screen or target and the longitudinal axisof the other two of said beams, each on opposite sides of said screen ortarget, being generally parallel to the other axis of said screen ortarget,

13. Means for directing two beams of electrons toward a plane, saidbeams being of such shape and so directed that each of the regions ofintersection of the beams with said plane is substantially rectangularand overlaps the other along the side thereof bounding said overlap tosubstantially said constant value at the opposite side of said overlapportion, the gradients being the same for both beams and being such asto give substantially constant intensity throughout said overlap portionand said adjacent portions.

14. A target, means for directing two beams of electrons toward saidtarget, said beams being of such shape and so directed that each of theregions of intersection of the beams with said target is substantiallyrectangular and overlaps the other along one side thereof by the sameamount throughout the overlap, the intensity of each beam at said targetbeing substantially constant throughout a considerable portion thereofadjacent said overlap portion and varying gradually from zero at theside thereof bounding said overlap to substantially said constant valueat the opposite side of said overlap portion, the gradients being thesame for both beams and being such as to give substantially constantintensity throughout said overlap portion and said adjacent portions,and means for generating a third beam of electrons and for causing it toscan said regions of intersection element by element.

15. In combination, an evacuated container having mounted therein amosaic target of light sensitive elements electrically insulated fromeach other and from a supporting electrode,

means in said container for generating a beam I of high velocityprimaryelectrons and for causing said beam to scan every elemental area in turnof a field on said mosaic, means for applying radiations from an objector field of view to said target, means within said container forcollecting secondary electrons emitted from said mosaic target whenimpacted by primary electrons from said beam, said primary electronshaving such velocity and number that said emission of secondaryelectrons leaves each element -immediately after it has been scanned bysaid beam of primary electrons at an equilibrium potential which ispositive with respect to the potential of said electron collectingmember, and electron generating means for applying low velocityelectrons to said mosaic in such quantity and at a velocity such thatthe potential of each of said elements is made negative with respect tothat of the collecting electrode, said last-mentioned means being soplaced with respect to the mosaic that the low velocity electronsapproach said mosaic at an angle with respect to the normal theretowhich has a cosine falling within the range between 0.9 and 1.0.

16. The combination of' elements as in claim 15 in which said lowvelocity electrons are continuously sprayed over the entire field onsaid mosaic target.

17. The combination of elements as in claim 15 in which said lowvelocity electrons are applied to said target from a plurality ofelectron guns, the axes of which are symmetrically placed around thenormal to the center of said field.

18. The combination of elements as in claim 15 in which said lowvelocity electrons are applied to said target from four electron guns,each generating a beam of substantially rectangular cross section at theplane of the mosaic target.

JOHN R. PIERCE.

